This invention relates to impedance measurements on transmission lines, and more particularly, to a method of performing such measurements using time domain reflectometry (TDR).
In measuring transmission line characteristics using time domain reflectometry (TDR), voltage pulses are applied to the line and line impedances or discontinuities are observed. The technique is used, for example, to locate major faults in a transmission line or cable. The ability to use this measurement technique is compromised, however, by a phenomenon known as "dribble up". The term describes the apparent upward drift in cable impedance as one moves further along the length of the transmission line. Heretofore, the phenomenon has been ascribed to high-frequency skin-effect losses in the line. See, for example, "Time Domain Reflectometry", Hewlett-Packard Application note #62, 1964; or, "Making Automated TDR Measurements", Handshake, Summer 1986 John McHugh, Tektronix.
Highly accurate line impedance measurements are often critical; the ability to maintain high speed data communication networks being but one example. To achieve this accuracy, however, "dribble up" effects must be eliminated from the test observations, regardless of the point along the length of the transmission line at which a measurement is made.